arXiv subjects

G. S. Da Costa

Publications and source records attributed to G. S. Da Costa.

At least 73 records · Page 4Linked to original sources

arXiv2008

A Spectroscopic Survey for Omega Centauri Members at and beyond the Cluster Tidal Radius

We have used the two-degree field (2dF) multi-fibre spectrograph of the Anglo-Australian Telescope to search for candidate members of the unusual globular cluster omega Centauri at and beyond the cluster tidal radius. Velocities with an accuracy of ~10 kms-1 were obtained for 4105 stars selected to lie in the vicinity of the lower giant branch in the cluster colour-magnitude diagram and which cover an area on the sky of ~2.4x3.9 deg2 centered on the cluster. Within the velocity interval 190-270 kms-1, the cluster member candidates have a steeply declining surface density distribution consistent with the adopted tidal radius of 57`. For stars in the sample beyond the tidal radius, an analysis of line-strengths from the spectra, as well as radial velocities, identifies only six stars as possible candidates for extra-tidal association with the cluster. If all six of these stars are indeed related to the cluster, then a maximum of 0.7 +/- 0.2 per cent of the total cluster mass is contained in the region between one and two tidal radii. Given this limit, we conclude that there is no compelling evidence for any significant extra-tidal population in omega Cen. The effects of tidal shocks on the outer parts of the cluster are consistent with this limit. Theories for the origin of omega Cen frequently suggest that the cluster is the former nucleus of a tidally stripped dwarf galaxy. Our results require that the stripping process must have been largely complete at early epochs, consistent with dynamical models of the process. The stripped former dwarf galaxy stars are therefore now widely distributed around the Galaxy.

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arXiv2007

ESO540-032: a Transition-Type Dwarf in the Sculptor Group

A color-magnitude diagram (CMD) based on HST/ACS data is presented for the Sculptor group dwarf galaxy ESO540-032. The CMD is dominated by an old red giant population but there is a small population of blue stars confined to the central regions of the galaxy as orginally noted by Jerjen and Rejkuba from ground-based data. From the luminosity of the red giant branch tip the distance is determined as 3.7 +/- 0.2 Mpc consistent with previous estimates. The mean metallicity is estimated as [Fe/H] = -1.7 +/- 0.2 from the color of the red giant branch. Isochrone fits indicate an age for the blue star population of ~100 Myr, perhaps less. New HI observations with the Australian Telescope Compact Array confirm that the dwarf contains approximately 10^6 solar masses of HI, centered on the optical image. The HI mass to blue luminosity ratio is 0.15, comparable to the Phoenix dwarf in the Local Group. These properties clearly confirm ESO540-032 as a transition-type dwarf in the Sculptor group.

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arXiv2007

Abundances on the Main Sequence of Omega Centauri

Abundance ratios of carbon, nitrogen and strontium relative to iron, calculated using spectrum synthesis techniques, are given for a sample of main sequence and turnoff stars that belong to the globular cluster omega Centauri. The variations of carbon, nitrogen and/or strontium show several different abundance patterns as a function of [Fe/H]. The source of the enhancements/depletions in carbon, nitrogen and/or strontium may be enrichment from asymptotic giant branch stars of low (1--3 solar masses) and intermediate (3--8 solar masses) mass. Massive rotating stars which produce excess nitrogen without carbon and oxygen overabundances may also play a role. These abundances enable different contributors to be considered and incorporated into the evolutionary picture of omega Cen.

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arXiv2007

The Stellar Populations of dE galaxies in nearby Groups

In this contribution initial results from colour-magnitude diagrams for five dEs in the M81 group are presented. The colour-magnitude diagrams, derived from HST/ACS images, reach well below the red giant branch tip and allow evaluation of distances and mean metallicities. Further, the intermediate-age upper-AGB stars seen in the diagrams allow estimates of the epochs of the last significant episode of star formation in the dEs. These epochs, and the relative numbers of upper-AGB to red giant branch stars, vary significantly from dE to dE. Preliminary inferences from similar HST/ACS data for five early-type dwarfs in the Sculptor group are also discussed.

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arXiv2006

A Sr-Rich Star on the Main Sequence of Omega Centauri

Abundance ratios relative to iron for carbon, nitrogen, strontium and barium are presented for a metal-rich main sequence star ([Fe/H]=--0.74) in the globular cluster omega Centauri. This star, designated 2015448, shows depleted carbon and solar nitrogen, but more interestingly, shows an enhanced abundance ratio of strontium [Sr/Fe] ~ 1.6 dex, while the barium abundance ratio is [Ba/Fe]<0.6 dex. At this metallicity one usually sees strontium and barium abundance ratios that are roughly equal. Possible formation scenarios of this peculiar object are considered.

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arXiv2006

Interstellar medium disruption in the Centaurus A group

We present the results of a 21 cm neutral hydrogen (HI) line detection experiment in the direction of 18 low luminosity dwarf galaxies of the Centaurus A group, using the Australia Telescope National Facility 64m Parkes Radio Telescope and the Australia Telescope Compact Array. Five dwarfs have HI masses between M_HI=4x10^5 to M_HI=2.1x10^7 Msol and 0.04 10^7 Msol or M_HI<10^6 Msol. This gap may be explained by the ram pressure stripping mechanism at work in this dense environment where all galaxies with M_HI<10^7 Msol have been stripped of their gas. The required intergalactic medium density to achieve this is ~10^-3 cm^-3.

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arXiv2006

The Age-Metallicity Relation of the SMC

The Small Magellanic Cloud (SMC) is the only dwarf galaxy in the Local Group known to have formed and preserved populous star clusters continuously over the past 12 Gyr. These clusters provide a unique, closely spaced set of single-age, single-metallicity tracers for a detailed study of the SMC's age-metallicity relation. Spectroscopic metallicity measurements, however, exist only for 6 of its clusters. Here we present metallicities for 7 additional SMC clusters based on Ca II triplet observations. The total sample comprises clusters spanning an age range of 12 Gyr. Complemented by age estimates from literature these objects provide us with a well-sampled, well-defined age-metallicity relation. We compare our first results with the chemical evolution history of the SMC in terms of a simple closed box model. We find that the general trend is well reproduced by this model.

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arXiv2006

The Age and Metallicity Relation of Omega Centauri

We present a metallicity distribution based on photometry and spectra for 442 omega Cen cluster members that lie at the main sequence turnoff region of the color-magnitude diagram. This distribution is similar to that found for the red giant branch. The distribution shows a sharp rise to a mean of [Fe/H] = -1.7 with a long tail to higher metallicities. Ages have then been determined for the stars using theoretical isochrones enabling the construction of an age-metallicity diagram. Interpretation of this diagram is complicated by the correlation of the errors in the metallicities and ages. Nevertheless, after extensive Monte-Carlo simulations, we conclude that our data show that the formation of the cluster took place over an extended period of time: the most metal-rich stars in our sample ([Fe/H] ~ -0.6) are younger by 2-4 Gyrs than the most metal-poor population.

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arXiv2005

Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452

We study the red giant populations of two dE galaxies, AM 1339-445 and AM 1343-452, with the aim of investigating the number and luminosity of any upper asymptotic giant branch (AGB) stars present. The galaxies are members of the Centaurus A group (D~3.8 Mpc) and are classified as outlying (R~350 kpc) satellites of Cen A. The analysis is based on near-IR photometry for individual red giant stars, derived from images obtained with ISAAC on the VLT. The photometry, along with optical data derived from WFPC2 images retrieved from the HST science archive, enable us to investigate the stellar populations of the dEs in the vicinity of the red giant branch (RGB) tip. In both systems we find stars above the RGB tip, which we interpret as intermediate-age upper-AGB stars. The presence of such stars is indicative of extended star formation in these dEs similar to that seen in many, but not all, dEs in the Local Group. For AM 1339-445, the brightest of the upper-AGB stars have Mbol~-4.5 while those in AM 1343-452 have Mbol~-4.8 mag. These luminosities suggest ages of approximately 6.5+/-1 and 4+/-1 Gyr as estimates for the epoch of the last episode of significant star formation in these systems. In both cases the number of upper-AGB stars suggests that ~15% of the total stellar population is in the form of intermediate-age stars, considerably less than is the case for outlying dE satellites of the Milky Way such as Fornax and LeoI.

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arXiv2005

An HST Study of the Supernovae Accompanying GRB 040924 and GRB 041006

We present the results from a {\it Hubble Space Telescope/ACS} study of the supernovae associated with gamma-ray bursts 040924 ($z=0.86$) and 041006 ($z=0.71$). We find evidence that both GRBs were associated with a SN 1998bw-like supernova dimmed by $\sim 1.5$ and $\sim 0.3$ magnitudes, respectively, making GRB 040924 the faintest GRB-associated SN ever detected. We study the luminosity dispersion in GRB/XRF-associated SNe and compare to local Type Ibc supernovae from the literature. We find significant overlap between the two samples, suggesting that GRB/XRF-associated SNe are not necessarily more luminous nor produce more $^{56}$Ni than local SNe. Based on the current (limited) datasets, we find that the two samples may share a similar $^{56}$Ni production mechanism.

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arXiv2005

The Nature of the Density Clump in the Fornax Dwarf Spheroidal Galaxy

We have imaged the recently discovered stellar overdensity located approximately one core radius from the center of the Fornax dwarf spheroidal galaxy using the Magellan Clay 6.5m telescope with the Magellan Instant Camera (MagIC). Superb seeing conditions allowed us to probe the stellar populations of this overdensity and of a control field within Fornax to a limiting magnitude of R=26. The color-magnitude diagram of the overdensity field is virtually identical to that of the control field with the exception of the presence of a population arising from a very short (less than 300 Myr in duration) burst of star formation 1.4 Gyr ago. Coleman et al. have argued that this overdensity might be related to a shell structure in Fornax that was created when Fornax captured a smaller galaxy. Our results are consistent with this model, but we argue that the metallicity of this young component favors a scenario in which the gas was part of Fornax itself.

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arXiv2005

The Stellar Populations of dSph Galaxies in Nearby Groups

In this contribution initial results are presented from a program to study in detail the stellar populations of dwarf Spheroidal (dSph) galaxies in three nearby groups. The long-term aim of the program is to assess the influence of environment in governing the evolution of these low-luminosity systems. Specific results described here include the detection and measurement of intermediate-age upper-AGB populations in dSphs in the M81 and Cen A groups, and the discovery that four of the five low-luminosity early-type dwarfs in the low density Sculptor group contain modest amounts of neutral hydrogen gas.

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arXiv2005

Abundances on the Main Sequence of the Globular Cluster Omega Centauri

Using the 2dF multi-fibre instrument on the AAT, moderate resolution spectra have been obtained for a large sample of stars on the main sequence and at the turnoff in the unusual globular cluster omega Centauri. We investigate the behaviour of CH, CN and SrII line strength indices as a function of overall abundance for the main sequence sample. A number of stars do not follow the relations defined by the majority. These anomalous objects can be categorised into (at least) three types. (1) Carbon enhanced stars, which represent about 5% of the sample, and which are found at all metallicities. Spectrum synthesis calculations show that the atmospheres of these stars are typically enhanced in C by factors of between 3 and 10. (2) Nitrogen enhanced stars, revealed for [Fe/H] > -1.3 by strong CN indices, which make up ~40% of the cluster main sequence population above this metallicity. These stars are enhanced in N by factors of up to 100. Our data, however, provide no constraints of their relative numbers at lower [Fe/H]. (3) Stars with enhancements of the s-process element Sr by factors of 30 to 60. The possible origins for these abundance anomalies are discussed.

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arXiv2005

The Absence of Extra-Tidal Structure in the Sculptor Dwarf Spheroidal Galaxy

The results of a wide-field survey of the Sculptor dwarf spheroidal galaxy are presented. Our aims were to obtain an accurate map of the outer structure of Sculptor, and to determine the level of interaction between this system and the Galaxy. Photometry was obtained in two colours down to the magnitude limits of V=20 and I=19, covering a 3.1 times 3.1 square deg area centred on Sculptor. The resulting colour-magnitude data were used as a mask to select candidate horizontal branch and red giant branch stars for this system. Previous work has shown that the red horizontal branch (HB) stars are more concentrated than the blue HB stars. We have determined the radial distributions of these two populations and show that the overall Sculptor density profile is well described by a two component model based on a combination of these radial distributions. Additionally, spectra of the Ca ii triplet region were obtained for over 700 candidate red giant stars over the 10 square deg region using the 2dF instrument on the Anglo-Australian Telescope. These spectra were used to remove foreground Galactic stars based on radial velocity and Ca ii triplet strength. The final list of Sculptor members contained 148 stars, seven of which are located beyond the nominal tidal radius. Both the photometric and spectroscopic datasets indicate no significant extra-tidal structure. These results support at most a mild level of interaction between this system and the Galaxy, and we have measured an upper mass limit for extra-tidal material to be 2.3 +/- 0.6% of the Sculptor luminous mass. This lack of tidal interaction indicates that previous velocity dispersion measurements (and hence the amount of dark matter detected) in this system are not strongly influenced by the Galactic tidal field.

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arXiv2005

The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda I and Andromeda III

(Abridged) We present the results of variable star searches of the M31 dwarf spheroidal companions Andromeda I and Andromeda III using the Hubble Space Telescope. A total of 100 variable stars were found in Andromeda I, while 56 were found in Andromeda III. One variable found in Andromeda I and another in Andromeda III may be Population II Cepheids. In addition to this variable in Andromeda III, another four variables are anomalous Cepheids. We found 72 fundamental mode RR Lyrae stars and 26 first-overtone mode RR Lyrae stars in Andromdea I giving mean periods of 0.575 day and 0.388 day, respectively. One likely RR Lyrae was not classified. For Andromeda III, 39 RR Lyrae stars are pulsating in the fundamental mode with a mean period of 0.657 day and 12 are in the first-overtone mode with a mean period of 0.402 day. While Andromeda I has a mean period that is consistent with its metallicity, the mean period of the fundamental mode RR Lyrae in Andromeda III is larger than expected. In Andromeda I, we found two RR Lyrae stars which are noticeably fainter than the horizontal branch. We discuss the possibility that these stars are associated with the recently discovered stellar stream in the halo of M31. We find =25.14+/-0.04 mag for Andromeda I resulting in a distance of 765+/-25 kpc. For Andromeda III, =25.01+/-0.04 mag giving a distance of 740+/-20 kpc. The relation between specific frequency of anomalous Cepheids and mean metallicity of the galaxy in the M31 dwarf spheroidal galaxies follows the same relation as the Galactic dwarf spheroidal galaxies.

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arXiv2004

A Wide-Field Survey of the Fornax Dwarf Spheroidal Galaxy

The results of a photometric survey of the Fornax dwarf spheroidal galaxy are presented herein. Accurate photometry in two colours (V and I) was collected over a 10 square degree area centred on the Fornax system with the aim of searching for extra-tidal structure. The data were complete to a magnitude of V=20, or approximately one magnitude brighter than the Fornax red clump stars. Stars were selected with a colour and magnitude lying near the Fornax red giant branch, thereby reducing contamination from the field star population. We were thus able to probe the outer structure of this dwarf galaxy. Immediately visible was a shell-like structure located 1.3 degrees NW from the centre of Fornax, approximately 30 arcmin beyond the nominal tidal radius at this position angle. We have measured the absolute visual magnitude of this feature to be M_V ~ -7. The feature is aligned with a previously reported shell (age approximately 2 Gyr) located near the core radius of Fornax. A statistical analysis of the extra-tidal stellar distribution further revealed two lobes situated on the Fornax minor axis which are aligned with the two shell-like features. The two-lobed structure combined with the two shells provide strong evidence that Fornax has experienced a merger event in the relatively recent past.

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arXiv2004

A Second Shell in the Fornax dSph Galaxy

In the search for tidal structure in Galactic satellite systems, we have conducted a photometric survey over a 10 square degree area centred on the Fornax dSph galaxy. The survey was made in two colours, and the resulting colour-magnitude data were used as a mask to select candidate Fornax RGB stars, thereby increasing the contrast of Fornax stars to background sources in the outer regions. Previously, we reported the presence of a shell (age 2 Gyr) located towards the centre of Fornax. In this contribution we reveal a second shell, significantly larger than the first, located 1.3 degrees NW from the centre of Fornax, outside the nominal tidal radius. Moreover, the distribution of Fornax RGB stars reveals two lobes extending to the spatial limit of our survey, and aligned with the minor axis and with the two shells. These results support the hypothesis of a merger between Fornax and a gas-rich companion approximately 2 Gyr ago.

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arXiv2004

Explaining the mysterious age gap of globular clusters in the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) has a unique cluster formation history in that nearly all of its globular clusters were formed either $\sim$ 13 Gyr ago or less than $\sim$ 3 Gyr ago. It is not clear what physical mechanism is responsible for the most recent cluster formation episode and thus the mysterious age gap between the LMC clusters. We first present results of gas dynamical N-body simulations of the evolution of the LMC in the context of its Galactic orbit and interactions with the SMC, paying special attention to the effect of tidal forces. We find that the first close encounter between the LMC and the Small Magellanic Cloud (SMC) about 4 Gyr ago was the beginning of a period of strong tidal interaction which likely induced dramatic gas cloud collisions, leading to an enhancement of the formation of globular clusters which has been sustained by strong tidal interactions to the present day. The tidal interaction results in the formation of a barred, elliptical, thick disk in the LMC. The model also predicts the presence of a large, diffuse stellar stream circling the Galaxy, which originated from the LMC.

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